1. Field of the invention
The present invention relates to the recording and playback of information on optical cards, discs or tape. More particularly, the invention relates to a method and apparatus for accurately recording both digital and variable length or analog pulses by real time monitoring of pulse initiation and termination, null inflective points, and further detecting and eliminating latent recording errors due to anomalies in the recording medium or due to the recording process itself.
2. Description of Background Art In U.S. Pat. No. 4,908,815, Mar. 13, 1990, the present inventors, Gregg, Shu and Wilson, disclosed an Optical Recording System utilizing Proportional Real-Time Feedback Control of Recording Beam Intensity. In that patent, the disclosure of which is hereby incorporated by reference into the present disclosure, a novel and highly effective system for accurately recording data via a real time feedback network to minimize errors in recording on an optical medium was disclosed.
A primary application for the prior-disclosed system is the reduction of recording errors in the recording of data on optical discs, of the type on which data is recorded by altering an optical property of a photo-sensitive or photo-reactive layer on the disc.
One commonly used type of optical disc usable in the prior-disclosed system has a reflective metallic coating overlain by a photo-sensitive or photo-reactive, transparent layer. On such discs, regions which contain no recorded data are highly reflective. Locations on the disc on which data have been recorded have reduced reflectivity, owing to the chemical or physical alteration of the photo-reactive layer overlying the reflective layer caused by the recording process.
As was described in detail in the prior disclosure and summarized below, the control of recording beam intensity by a closed-loop feedback control system previously disclosed by the present inventors provides an effective means for reducing recording errors to substantially lower levels than were obtainable with prior art systems. However, certain gross chemical or physical anomalies may exist in the surface or grooves of an optical disc to be recorded. Thus, microscopic pits, deformed grooves or inclusions in a recording surface may be beyond the capabilities of the prior-disclosed error minimization system to compensate thereby resulting in some residual recording errors.
The present invention, as disclosed herein, is intended to provide a new method and apparatus for accurately recording variable length pulses such as analog data and for reducing residual errors in recording on an optically sensitive medium. Such reduction of errors should be so complete that not only digital may be recorded and read, but also wide band analog recording can be accomplished based on the burst length of pulses recorded as the recording beam passes along the recording track, thereby enabling an entirely new dimension in the recording and playback of information on optical discs, heretofor unknown and undiscovered.
The invention discloses a process and apparatus that permits the accurate writing and reading of pulses of varying width by means of continuous, real-time control of the write power by taking into account that said power is finite and that there may be a variety of thresholds and sensitivities in the photosensitive media employed, as well as physical or chemical anomalies or variations within a given medium for any optically recordable disc or tape surface.
In return-to-zero (RZ) recording, only the presence or absence of recorded pits need be read; however, the more widely used non-return-to-zero (NRZ) recording methods may employ a multiplicity of contiguous "highs" in various combinations resulting in combined "on" pulses of integrally stepped widths. For example, in contemporary Compact Disc (CD,) format of digital audio discs, recorded pulses may vary between 0.9 and 3.3 micrometers (um) long, corresponding to between 3 and 11 data bits or data elements, respectively, in steps of 0.3 um long. Correct reading of the number of data bits per pulse is therefore required, such as parity bit error detecting schemes, otherwise an elaborate, band-width-consuming error correction code scheme is necessary. Similarly, in another format (Laser Vision,) for combined video and audio duplication and recording in real-time, signals are frequency-modulated (FM) during mastering (duplicating) and during writing (recording) on recordable discs as well as on optical tapes. In either case, considerable digital data are necessary in the vertical blanking interval section of the disc in addition to digital audio channels.
In the case of Laser Vision, limited error reduction may be achieved with high modulation frequencies but at the expense of bandwidth. In addition, extreme care must be provided during each step of replication of the disc. In existing, smaller, portable, direct recording and playback of digital and FM signals, lower levels of precision and lower costs usually prevent, among other measures, the use of externally modulated continuous lasers of relatively short wavelengths, presently utilized in conventional disc mastering processes. Laser diodes, in small or portable optical recorders or duplicators, are virtually mandated due to their simplicity, internal modulability, efficiency, small size and low cost. However, weighing against these advantages are the disadvantages of lower optical output power and longer wavelengths involved with use of laser diodes. Existing laser diode wavelengths approximate the length of a minimum bit or data element to be recorded, in the realm of 1 um, and places the system well into an optical diffraction limited region where beam wavelength and minimum recordable data element size very nearly conform. Although laser diodes exhibit fast rise and decay times, the power limitations, both peak and average, may impose severe restrictions on the bandwidths to be written unless novel means are employed.
In order to understand the novelty of the invention, it is necessary to more particularly review the current practice of optical recording technology. Assuming that the photosensitive medium to be recorded on is stationary with respect to the write means, a write pulse (spike) of unitary amplitude is fed to the recording system, the write pulse width being that which, when converted to optical power, will produce a write beam of a specific diameter. Said diameter is taken to be the customary width of a Gaassian beam, essentially equal to the beam's wavelength and which encompasses over 80% of the beam power.
For sake of simplicity, it is assumed that a constant power density per unit area is provided within the beam diameter on the reflective photosensitive surface. It is further assumed for sake of simplicity of explanation that the width of the write pulse is of such amplitude and duration that the resulting thermal energy will effect a pit of 1/4 wavelength (.lambda.) in depth which causes a destructive interference pattern and therefor minimum reflectivity. The pit is produced by the heat-absorption of the photosensitive surface but may comprise any optical effect which will yield a minimum reflectivity when an ideal data element is recorded.
In a conventional embodiment of the optical disc discussed herein a highly reflective coating is applied to the photosensitive layer which, in turn, is supported by a transparent substrate. The reflective layer is then covered by a compliant protective layer. The recording spike creates a reaction in the optically active layer causing a curved deflection bending, or pitting of the reflective layer. For minimum reflection, the deflection or pit depth is in the order of 1/4 wavelength of the laser beam and creates a truncated conical or approximately cylindrical pit effect. In the production of the truncated conical approximately cylindrical pit, the duration of the recording spike was so short that it was assumed that the recordable medium was essentially stationary, hence the center of the pit conforms in displacement and time with the beam spike. However, this assumption cannot hold for pulses longer than a spike since the recordable surface is not stationary with respect to the beam.
If, on the other hand, it is desirable to record information in an optical medium that employs other than digital recording techniques it would be desirable to form an elongated pit of variable length in addition to approximately cylindrical pits. However, before proceeding with the mechanics of a prolonged pit, or slot, it is necessary to demonstrate how the approximately cylindrical pit is read. For an approximately cylindrical pit to be produced by a spike occurring at a time along the optical path corresponding to zero "0" displacement, the pit will have an effective diameter of approximately one wavelength and will be aligned with and centered the laser beam. The cylindrical wall of the pit, spanning a diameter of one wavelength will be sloping evenly and symmetrically. Now on reading the recorded pit, the leading edge of the read beam, also of one wavelength, at a half wavelength of displacement in advance of the pit center, begins to approach and to detect the drop in reflected energy provided by the gradual slope of the pit wall, i.e. there is a gradual decrease in reflection. The reflected energy drops to a narrow null at the pit center (a depth of 1/4 wavelength) and then gradually rises symmetrically at the other side of the pit wall to the fully reflective recording medium surface level, outside the pit. Therefore, detection of the pit walls per se is relatively indeterminate when practical considerations are taken into account. Only detection of an instantaneous null inflection point at the center of the pit can provide accurate information of pit location and therefore timing.
Again, considering a unity (100%) amplitude pulse of one wavelength in duration, a pit would be formed with the rise of the pulse at zero displacement. However, the write beam, held at that constant power level throughout progresses synchronously with the pulse as a function of time along the recording track and imparts cumulative and therefor excessive beam energy across the pit diameter. At the intersection of the centerline of the optical path with the nearly distal edge of a pit or slot, there accumulates an excess of energy approaching 400% more than that which initiated pit formation. The excess energy trails off to zero at the distal end of the pit or slot. The end result of this phenomenon may be a "teardrop" effect often observed under a microscope after writing a data element. In the case of a reflective disc, in which the accuracy of differential displacement of the reflective layer is critical, it is impossible for a flat-bottomed pulse and a corresponding optical footprint of constant amplitude write-beam to yield an ideal flat-bottomed slot between initial and terminal edges of the pit; i.e. the teardrop effect not only tends to enlarge pit width but also pit depth. In the intermediate stage in the progression of the write-beam from one side of the pit or slot to the other side, cumulative excess beam energy is experienced by the photosensitive layer, which further expands and drives the reflective layer in the process. This process creates a downward sloping ramp of an optical footprint. The end result is that, mainly in the downward ramp, part of the reflective layer in the bottom of the slot may be driven past the non-reflective phase and even into another reflective phase, or beyond, depending upon the optical characteristics of the medium.
A substantial number of prior art references disclose methods for detecting defective regions of an optical recording medium on a substrate. Those references known by applicant consist of the following U.S. Pat. Nos.:
4,197,011, Hudson, Apr. 8, 1980, Defect Detection and Plotting System; PA0 4,352,564, Roach, Oct. 5, 1982, Missing Order Defect Detection Apparatus; PA0 4,412,743, Eberly, Nov. 1, 1983, Off-Axis Light Beam Defect Detector; PA0 4,464,050, Kato, et al., Aug. 7, 1984, Apparatus For Detecting Optically Defects; PA0 4,505,585, Yoshikawa, Mar. 19, 1985, System For Detecting Defects On An Optical Surface PA0 4,508,450, Oshima, Apr. 2, 1985, System For Checking Defects on A Flat Surface Of An Object; PA0 4,541,716, Crooks, et al., Sep. 17, 1985, Detection of Defects In A Circular Or Spiral Diffraction Grating; PA0 4,693,608, Kitagawa, Sep. 15, 1987, Method And Apparatus For Determining Position Of Points On An Article; PA0 4,794,264, Quackenbos, et al., Dec. 27, 1988, Surface Defect Detection And Confirmation System And Method; PA0 4,794,265, Quackenbos, et al., Dec. 27, 1988, Surface Pit Detection System And Method; and PA0 4,832,487, Mikuriya, et al., May 23, 1989, Test System For Optical Discs.
None of the above-cited prior art references, or any other references that the applicant is aware of, discloses an analog recording process or a residual error reduction method for optical recording errors detected in real time during the recording process, and the location of the improperly recorded data, along with correct data, stored in a different location on an optical disc. Moreover, none of the prior art references known to the present applicant discloses the use of an error signal, from an open or closed servo loop controlling a recording light beam, to detect defective recording regions prior to, during and after recording each location on an optical disc. In addition, no references of which applicant is aware is disclosed herein which permits the degree of accuracy in optical data track pulse initiation and termination required such that analog wide band recording in addition to existing digital recording art is practical.